The use of a type of enzyme known as a "pectinesterase" (EC 3.1.1.11; systematic name "pectin pectylhydrolase"; also known as "pectin esterase", "pectin methylesterase", "pectin methoxylase" or "pectin demethoxylase"; abbreviated hereafter as PE) for causing gelling or increase of viscosity of aqueous media containing certain pectins is well known. By way of example, WO 94/12055 (Gist-Brocades N.V.) and WO 94/25575 (Novo Nordisk A/S) both describe, inter alia, foodstuff-related applications of PE-catalyzed gelling or viscosity increase of pectin-containing media.
Moreover, certain pectins (hereafter often referred to as "phenolic pectins"), notably pectins obtainable from members of the plant family Chenopodiaceae (which includes beets and spinach), as well as hemicellulosic material from some cereals (e.g. from wheat and maize), are substituted to some extent with substituents derived from certain carboxylic acids containing phenolic hydroxy groups. These phenolic substituents are often derived from substituted cinnamic acids, and in the case of, e.g., phenolic pectins the substituents in question are often "ferulyl" functionalities, i.e. ester functionalities derived from "ferulic acid" (4-hydroxy-3-methoxycinnamic acid; it does ot appear to have been established clearly whether "ferulic acid" embraces cis or trans isomeric forms, or both).
With respect to the gelling or increase of viscosity of aqueous media containing such phenolic pectins (and some related phenolic polysaccharides) by processes which do not involve the use of PE, the following may be mentioned:
J.-F. Thibault et al., in The Chemistry and Technology of Pectin, Academic Press 1991, Chapter 7, pp. 119-133, describe the oxidative cross-linking of beet pectins (in connection with the gelling thereof) by purely chemical modification, using a powerful oxidant such as, e.g, persulfate. With respect to enzyme-catalyzed processes, the Thibault et al. reference also describes the gelling of sugar beet pectin using a combination of a peroxidase and hydrogen peroxide.
FR 2 545 101 A1 describes a process for modification (including gelling) of beet pectin involving the use of "an oxidizing system comprising at least an oxidizing agent and an enzyme for which the oxidizing agent in question is a substrate". The only types of oxidizing agent and enzyme which are specified and/or for which working examples are given are hydrogen peroxide and peroxidases, respectively.
Similarly, WO 93/10158 describes gelling of aqueous hemicellulosic material containing phenolic substituents [e.g. substituents derived from ferulic acid (vide supra)] using an oxidizing system comprising a peroxide (such as hydrogen peroxide) and an "oxygenase" (preferably a peroxidase).
Applicant's co-pending PCT application No. PCT/DK95/00317 (unpublished at the time of filing of the present application) discloses a method for causing gelling or increase of viscosity of an aqueous medium containing a gellable polymeric material having substituents with phenolic hydroxy groups (such as a phenolic pectin as mentioned above), the method comprising adding an oxidase to the aqueous medium. The oxidase enzymes in question (which are generally classified under EC 1.10.3) are oxidoreductases (EC 1) which are capable of catalyzing oxidation of phenolic groups and which employ molecular oxygen as acceptor. Preferred oxidases in the context of the invention disclosed in PCT/DK95/00317 are laccases (EC 1.10.3.2).
Phenolic pectins of the above-mentioned types, which are preferred starting materials in the context of the present invention, and which--as already mentioned--are naturally occurring phenolic polysaccharides, are readily available relatively cheaply and are of proven physiological safety with regard to ingestion by, and contact with, humans and animals.